Temperature rate control system

ABSTRACT

A temperature rate control system for a gas turbine having sensors in the path of the exhaust gasses which produce an electrical signal proportional to temperature. A reference circuit provides a signal proportional to maximum allowable temperature which opposes the feedback signal at a summing junction. A third signal applied to the summing junction in opposition to the reference signal and varying toward zero as a function of time results in the summing junction output signal which varies the turbine fuel input inversely whereby the temperature increase rate of the turbine is held to a predetermined value.

United States Patent 60/3928 X 415/15 X 415/17 X s, James C ell, JosephB.

3,226,558 12/1965 Wa1ker...........

3,291,146 12/1966 Wa1ker.........................

Primary Examiner-Clarence R. Gordon Attorneys-John B. Sponsler, GeraldR. Wood Davis, Frank L. Neuhauser, Oscar B. Wadd Forman and Arnold E.Renner v. n a P m u .E c v .I m. 9 w a K w n dks r 3 mi em mag nan IGMUE RR8 AG r 0. de m mm .c mmem V p MS .m AFPA 11]] 2 253 7 2247 .l[.llil.

[54] TEMPERATURE RATE CONTROL SYSTEM ABSTRACT: A temperature ratecontrol system for a gas turbine having sensors in the path of theexhaust gasses which proportional to temperature. A rence circuitprovides a signal prop allowable temperature which opposes th summingjunction. A third signal a produce an electrical signal refe tion inopposition to the reference si zero as a function of time results in mmm Fm m M m m Tm n t w L f .1 0 E d C M 5U .mF l. 1]] 2 O 6 5 55 5 [[1put signal which varies the turbine fuel input inversely whereby thetemperature increase rate of the turbine is held to e m a V d e .m m m ed m P a DO XX 300 42 H9 O m O 6 2,971,337 2/1961Wintrode..................... 3,082,954 3/1963 Offner..........3,295,316 .1/1967 Beatrice 3 7 COMMUN l l l 1 ma n 1 vi:

l TEMPERATURE? LFEE TEU llilllilli li SUPPRESSION L CIRClIIl TEMPERATUREL BACKGROUND OF THE INVENTION To obtain the greatest efficiency of thegas turbine, it is desirable to operate with the temperature of thegases entering the turbine section as high as possible. However, inorder to operate within design stress limitations of the turbine parts,there is a maximum allowable temperature which should not be exceeded.Thus, a temperature control system is required which will override thespeed or the load control of the turbine should the turbine inlettemperature exceed a preset limit. Since it is impractical to measureturbine inlet temperature directly, the temperature control system isdesigned to control the turbine inlet temperature indirectly, bycontrolling exhaust temperature. During normal steady state operation ofthe gas turbine up to 100 percent of turbine speed, exhaust temperatureis held at an optimum value by the fuel level control function. Duringthe starting sequence of the turbine, the temperature control is anintegral part of the starting function. In the starting sequence whenthe turbine is fired, the flame detectors initiate a timing periodduring which the temperature of the turbine is gradually increased toits final running temperature. In prior art the control circuitry whichperforms this time function includes a motor operated rheostat orhydraulic function which gradually moves through a predetermined numberof steps whereby the fuel applied the turbine is controlled as afunction of time by means of a rheostat or hydraulic system or someother mechanical means. The adjustment of the upper and lower limits offuel level applied to the turbine is generally a mechanical adjustmentwithin the control and usually difficult to perform. The rate at whichthe temperature is allowed to increase over the time period is generallynot adjustable due to predetermined motor speed in the rheostat andhence, is difficult to alter.

SUMMARY OF THE INVENTION Toovercome these difficulties the presentinvention provides for a control wherein a variable reference signal istested against a feedback signal representing the temperature of theexhaust gas of the turbine. The reference signal being adjustable as afunction of time causes a controlled allowable temperature increasewithin easily adjustable limits to occur over a predetermined timeperiod. This time period is adjustable without affecting either theupper or lower limits of temperature set by the control.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. ll shows the temperature controlin relation to other functional control used in turbine fuelapplication.

FIG. 2 shows the temperature rate control having a suppression circuitincluded for controlled startup of the turbine.

DETAILED DESCRIPTION Refer now to FIG. 1 wherein the control system forrunning a gas turbine engine has three main parameters of control: (1)startup, (2) speed and (3) temperature. Into these three parameters maybe fed a number of other parameters, such as acceleration, as well asseparate input signals such as the load input on the turbine. The figureis a block diagram showing these three main parameters wherein thestartup control 111 has an output connected to minimum value gate 113through diode 112. The temperature control 115 similarly has its outputconnected to minimum value gate 13 through diode l4; and the speedcontrol 117, which includes acceleration control, also has its outputconnected to minimum value gate 13 through diode 16. The minimum valuegate provides an input signal to the amplification circuitry 119 whichin turn has an output connected to the fuel control circuitry (notshown) of the gas turbine. The temperature control is an intricate partof the entire turbine control and therefore must be shown in conjunctionwith the other two major parameters, startup and speed control. Thefunction of the startup control in the initial phase of the startup ofthe turbine is to withhold fuel from the turbine until a predeterminedfiring speed has been reached; at that two signals is zero, the machineis point the startup control provides a signal which initiates thefiring of the fuel. Immediately upon detection of the flame, the fuellevel input is adjusted automatically for a warmup period wherein theturbine is gradually brought up to temperature in order to avoid athermal shock to the hot gas path parts. At the end of the warmupperiod, the startup control initiates an accelerate period which causesthe fuel control to provide more fuel to the gas turbine. The gasturbine continues to accelerate with the increase of fuel suppliedthereto until the rate of temperature increase intercepted by a ramprate of allowable temperature rise which is a preset value allowing fora predetermined increase in temperature per unit length of time of theexhaust gasses of the turbine, the rate of which is a predeterminedvalue for bringing the gas turbine from the warmup temperature level tomaximum temperature level. Since a gas turbine efficiency increases withspeed, the gas turbine begins to accelerate faster and accelerationcontrol will assume control of the turbine causing fuel consumption tobe cut back. Thus, the speed of the turbine will increase at the rateallowed by the acceleration control. When the operating speed has beenreached, the speed control assumes responsibility of running theturbine. Generally, the turbine is provided with a set of magneticpickup units which are mounted on the shaft and provides a pulse trainhaving a frequency of pulses which is proportional to the speed of theturbine. An analog voltage signal provides the reference signal againstwhich the magnetic pickup signal is tested. When the reference inputcancels the feedback input, in other words, when the resultant of therunning percent of reference speed. The third main parameter, thetemperature control, has the purpose of limiting the startup fuel inputso that operating temperatures of the gas turbine are maintained at safevalues, using exhaust temperature as a prime input signal. Thetemperature control provides an overriding signal such that temperatureincrease of the exhaust gasses may, for example, increase at not morethan 5 per second. lfthen fuel input is at a rate whereby thetemperature increase of the exhaust gasses will exceed this limit, thetemperature control will override all of the other portions of theturbine control and limit the fuel input to the turbine.

Referring now to FIG. 2 the temperature control circuit is divided inthree major functions, a temperature reference circuit 211, atemperature feedback circuit 23 and a suppression circuit 25. The outputsignals of each of these divisions is applied to a summing junction 54wherein the output signals are algebraically added to form theinput ofan operational amplifier 45 which in turn directs control circuitry ofthe fuel pump for the gas turbine (not shown). The performance of theoperational amplifier is well known to those skilled in the art, henceonly the symbolic representative thereof is shown.

The temperature reference circuit 211 includes a variable resistance 47which on one side connects to the negative bus 49 and on the other sideis connected to summing junction 54.

The temperature feedback circuit 23 receives a signal from temperaturesensor 52 and applies this signal through resistor 51 to summingjunction 54.

The suppression circuit 25 consists of a potentiometer 33 connected atone end to positive bus 31. At the other end, potentiometer 33 connectsto the common bus 37 and to the anode of diode 35. The slider connectionof potentiometer 33 feeds current to one side of capacitor 4.1 andthrough switch 43 to the other side of the capacitor 41, the cathode ofdiode 35, the collector of transistor 46 and through resistor 39 tosumming junction 54. The emitter of transistor 46 connects throughresistor 57 to switch 59 and through resistor 61 to the negative bus 49.Switch 59 connects to negative bus 49. The common bus 37 connects to thebase of transistor 46 through resistor 53 and is connected throughresistor 55 to the negative bus 49. Switches 43 and 59 are symbolic andmay be any circuit closing device such as relays or transistor switchesdepending on the requirements of the application of the circuit.

The operation of the temperature control circuit without the suppressioncircuit 25 provides that the reference circuit 21 produces a negativesignal which is applied to summing junction 54. This signal opposes thepositive temperature feedback circuit 23 at summing junction 54. Whenthe reference signal exceeds the feedback, the operational amplifyingcircuit 45 has a large positive output, causing the turbine to run at aspeed determined by the speed control circuit 17. If the temperaturefeedback signal is more positive than the temperature reference signaldue to excessive heat of the exhaust gasses, the summing junction 54tends to become more positive which due to the inverting characteristicsof the operational amplifier 45 causes a more negative signal to beproduced thereby, which when applied to the fuel control (not shown)causes a reduction in fuel supply and temperature of the turbine. If thesignal from reference circuit 21 is such that the summing junctionbecome more negative, the resultant output signal of the operationalamplifier 45 will be more positive causing increased fuel supply whichin turn allows an increase in temperature of the turbine. The minimumvalue gate in FIG. 1 provides means for allowing that function which hasthe lowest output signal to assume control. The gate normally has apositive polarity through a connection to the positive bus 31 throughresistor 18. If excessive temperature causes the output of thetemperature control to become more negative, current will flow from theminimum value gate 13 through diode 14 into the temperature control 15,thereby diverting some of the current which would normally be directedthrough the amplification function 19 to the fuel control (not shown).

With temperature control 25 in FIG. 2 in the circuit, at star-- tup ofthe turbine, switch 43 of the suppression circuit 25 is closed causing apositive voltage from the voltage dividing potentiometer 33 to beapplied through resistor 39 to the summing junction 54. This causes thesumming junction 54 to tend to assume a positive polarity, resulting ina more negative output from the operational amplifier 45. This resultsin a suppressed allowable temperature. At the time the turbine is fired,switch 43 is opened and capacitor 41 which up to this moment wasdischarged begins to charge negatively through a constant current sourceconsisting of resistors 61 and 57 and transistor 46 at a predeterminedrate. This rate may be changed by closing switch 59 whereby the effectof resistor 61 is removed from the circuit. As the capacitor 41 charges,the voltage level at the cathode of diode 35 is reduced linearly towardzero due to the constant current source characteristics of transistor46. The summing junction tends to become more negative, which in turncauses the output of the operational amplifier 45 to become morepositive, resulting in increased fuel supply and a correspondingincrease in allowable turbine temperature. When the capacitor 41 isfully charged, the turbine temperature limit will be determined by thecombination of the negative output signal of the temperature referencecircuit 21 and the positive output signal of the temperature feedbackcircuit at summing junction 54. The gas turbine thus has a startingcycle which provides minimum startup time with a minimum of temperatureshock. Temperature rate may be allowed to increase by reducing theresistance in the capacitor charging path, resulting from the closing ofswitch 59. This allows the capacitor 41 to charge at a faster rate,causing the bias current at the summing junction to reduce at a fasterrate, which in turn provides positive going signal to the fuel controlthereby allowing fuel to be applied to the turbine at a faster rate.

Adjustment of potentiometer 47 in the temperature reference circuit 21will change the level of negative current introduced to summing junction54 thus changing the level of positive voltage from the temperaturefeedback circuit 23 which is required to overcome this and therebychanging the upper temperature limit of the turbine, without changingthe rate of temperature increase set by suppression circuit 25.Similarly, adjustment of potentiometer 33 of the suppression circuit 25will change the lower limit of the temperature suppressipn by changingthe initial voltage condition of capacitor 41, without changing the rateof charge thereof and as a consequence without changing the rate oftemperature increase of the turbine.

While the invention has been explained and described with the aid ofparticular embodiments thereof, it will be understood that the inventionis not limited thereby and that many modifications retaining andutilizing the spirit thereof without departing essentially therefromwill occur to those skilled in the art in applying the invention tospecific operating environments and conditions. It is thereforecontemplated by the appended claims to cover all such modifications asfall within the scope and spirit of the invention.

What is claimed is:

l. A gas turbine fuel control system including means for producing acontrol signal operable to control the temperature increase rate of theturbine comprising:

a. first circuit means for producing a feedback signal proportional toturbine exhaust gas temperature;

b. second circuit means for providing a limit signal representative ofthe maximum allowable exhaust gas temperature;

c. means to govern the rate of rise of exhaust gas temperature includingthird circuit means operable to generate a,

time varying reference signal; and

d. means to sum said feedback, limit and reference signals and toproduce a control signal representative of permissible turbinetemperature.

2. The invention in accordance with claim 1 in which the time varyingreference signal varies linearly with respect to time from apredetermined maximum value towards zero.

3. The invention in accordance with claim 1 including means to vary themaximum value of said reference signal without varying the rate at whichsaid reference signal varies.

4. The invention in accordance with claim 1 including addi tional meansto change the rate at which said reference signal varies independentlywithout changing the maximum value of

1. A gas turbine fuel control system including means for producing acontrol signal operable to control the temperature increase rate of theturbine comprising: a. first circuit means for producing a feedbacksignal proportional to turbine exhaust gas temperature; b. secondcircuit means for providing a limit signal representative of the maximumallowable exhaust gas temperature; c. means to govern the rate of riseof exhaust gas temperature including third circuit means operable togenerate a time varying reference signal; and d. means to sum saidfeedback, limit and reference signals and to produce a control signalrepresentative of permissible turbine temperature.
 2. The invention inaccordance with claim 1 in which the time varying reference signalvaries linearly with respect to time from a predetermined maximum valuetowards zero.
 3. The invention in accordance with claim 1 includingmeans to vary the maximum value of said reference signal without varyingthe rate at which said reference signal varies.
 4. The invention inaccordance with claim 1 including additional means to change the rate atwhich said reference signal varies independently without changing themaximum value of said signal.
 5. The invention in accordance with claim4 in which said additional means comprises a series arrangementincluding a capacitor, a semiconductor device and a plurality ofresistors and further including means to selectively render a portion ofsaid resistors ineffective.